Engines may be operated using boosting devices, such as turbochargers or superchargers, to increase mass airflow into a combustion chamber. Turbochargers and superchargers compress intake air entering the engine using an intake compressor. Further, one or more intake charging devices may be staged in series or parallel to improve engine boost response.
One example of a multi-staged boosted engine is shown by Stewart et al. in U.S. Pat. No. 7,958,730. Therein, a multistage series turbocharger apparatus contains a low pressure turbocharger and a high pressure turbocharger, coordinated to maintain at least one operating parameter by adjusting turbocharger speeds. The low pressure turbocharger allows for quick acceleration to compensate for the slow acceleration of the high pressure turbocharger, also known as turbo lag. The compressor speed of either turbocharger may be regulated by opening and closing a corresponding wastegate.
However, the inventors herein have recognized potential issues with such systems. As one example, the regulation of turbocharger boost pressure and turbocharger speed by wastegate actuation may be slow, resulting in boost errors. In one example, if the compressor of a turbocharger overshoots a desired compressor speed, a wastegate coupled to the corresponding turbine may be opened to reduce exhaust flow through the turbine. However, due to the relatively slow turbocharger dynamics of the wastegate, the desired boost pressure may decrease faster than the compressor can be decelerated. Consequently, while the turbine is decelerating, a boost pressure downstream of the compressor may exceed a desired throttle inlet pressure, resulting in excessive engine torque delivery. In some examples, an engine throttle may be used to address the boost pressure since throttle adjustments have an almost immediate impact on boost pressure. However, the faster dynamics of the throttle may confound the wastegate control loop.
The inventors herein have recognized that an electric boost provided by an electric motor-driven supercharger compressor can have a substantially immediate impact on boost pressure in a staged engine system. In particular, at lower engine airflows, such as idle airflows, a transient positive boost pressure can be provided by rotating the supercharger compressor via the electric motor. This enables turbo-lag for a downstream turbocharger compressor to be reduced. In addition, at higher engine airflows, such as when an intake aircharge is boosted by the downstream turbocharger compressor, airflow into the downstream turbocharger compressor can be limited, or choked, by rotating the supercharger compressor via the electric motor, the choked flow determined by the rotation speed of the supercharger compressor. Thus in one example, the issues described above may be addressed by a method for a boosted engine, comprising: bypassing a first, upstream compressor and providing a flow of compressed air to a piston engine via a second, downstream compressor and, in response to a boost pressure overshoot, adjusting speed of the first compressor. In this way, boost pressure overshoot experienced at a downstream turbocharger compressor may be reduced by operating an upstream supercharger compressor as a flow restrictor.
As one example, an electric supercharger including a compressor driven by an electric motor may be staged upstream of a turbocharger including a compressor driven by an exhaust turbine. To reduce turbo lag, while the turbocharger compressor spins up, the electric supercharger may be transiently operated to provide positive pressure. Then, when the turbocharger compressor is spinning and providing boosted aircharge, in response to a boost pressure overshoot, the electric supercharger may be accelerated to choke the flow of air into the turbocharger compressor, and thereby into the engine. As such, for a given supercharger compressor speed, there may be a corresponding effective choke line that allows a specific amount of airflow. Thus, the engine controller may operate the electric supercharger at a speed to provide an airflow to the downstream turbocharger that reduces the boost pressure overshoot.
In this way, airflow into a turbocharger compressor can be substantially immediately restricted, allowing for expedited boost overshoot control. The technical effect of choking intake airflow to a downstream turbocharger using an upstream electric supercharger is that airflow to the turbocharger can be more precisely regulated to a level desired based on driver demand. This enables boosted air pressure to be regulated more quickly, and excess engine torque delivery to be reduced. Further, the supercharger based choked airflow may be provided in coordination with throttle adjustments and in the complementary frequency band as wastegate adjustments, allowing for a faster and more accurate regulation of the boost pressure. Further, the wastegate control loop may be tuned more aggressively since the fast dynamics of the electric supercharger would be able to damp any oscillations and reduce the boost pressure overshoot.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.